This invention relates to tape speed control apparatus. It relates more particularly to a signal generator which can be incorporated easily into a tape transport to provide output signals indicative of the true tape speed that can be used to control the tape drive motors.
In magnetic tape transports of the type with which we are concerned which drive the tape reel to reel in a cassette, the tape should move at substantially constant speed between the let-off reel and the take-up reel. Heretofore, it has been the practice to control tape speed by controlling the current applied to the reel drive motors in accordance with timing signals prerecorded on a tape timing track. Since the magnetic tape is relatively narrow, the required presence of the timing track has meant that one less track is available for recording data.
Of course one answer is to make the tape wider so that it can accommodate both the timing track and one or more extra data tracks. However this solution requires, in addition, wider reels, a non-standard cassette and a wider read/write head to accommodate the wider tape, all of which add to the overall size and cost of the tape transport.
It has also been proposed to use signals from a tape driven tachometer or encoder in lieu of the timing track signals to control the reel drive motors in various web handling apparatus including devices that transport tapes between large reels. In such apparatus, the tape or web moving from one reel to the other is engaged around a rotary drive shaft or wheel connected to a rotor in the encoder. Rotation of the rotor causes the encoder to produce output signals indicating the speed of the tape. In order for the speed indication to be accurate, however, there must be no slippage between the encoder drive wheel and the moving tape. For this reason, it has been the practice theretofore to make the drive wheel relatively large in diameter and to provide a considerable amount of tape wrap around the wheel. This maximizes the surface contact between the tape and wheel and thus minimizes slippage between the two. That arrangement is not feasible, however, in cassette tape transports. This is because the tape that must be engaged by the encoder drive wheel is contained entirely within the cassette housing. Therefore the drive wheel must engage the tape through an opening in the edge of the cassette which because of various constraints must be of limited size. Also the wheel can only project into the cassette a fraction of an inch so that the amount of tape wrap about the drive wheel is very limited. Consequently there is slippage between the wheel and the tape so that the output of the encoder does not accurately reflect the speed of the tape.
For these reasons the industry has tended to retain the tape timing track to control tape speed in cassette type transports and thus forego the ability to store more data on a given width tape.